Avoid Re-Placing the Same Parts Over and Over Again with a Pre-Placement Checklist

Sometime during the evolution of the automobile, car makers decided that reading and understanding engine gauges were too difficult tasks for the average driver. To remedy this they substituted the check engine light instead; unlike the malfunction indicators of today which are computer controlled warnings, those early check engine lights would only tell you of a problem once it was occurring. Reporting a problem after it occurs is not the purpose of a check, which is why the check engine light in those older cars was such a joke. The point of a good checking procedure is to catch problems before they happen. This is why we use checklists in our daily lives, to look for potential problems or to make sure that we don’t forget something important.

The only real purpose of the check engine light in those days was simply to tell you; “check the engine, because it needs to be replaced.” But checklists can be a much larger help when you design printed circuit boards. A checklist will give you the assurance that you have completed one phase of the design and are ready to move on to the next. Having a pre-placement checklist is a great example of this: you can check to make sure that your library and schematic are ready for placement, and if you have enough placement information to proceed. Checking to make sure that you are prepared for beginning component placement is the best way of protecting yourself from the pain of re-placing those parts later on.

First Step for Your Pre-Placement Checklist: Is the Library Complete?

I have seen many designers rush into placement without being ready only to run face-first into incomplete or incorrect library parts. As one of the foundations of a PCB design, an incorrect component footprints could make an entire design suffer. With today’s high-density designs, space is at a premium. Since changing a footprint often results in it getting bigger instead of smaller, correcting an incorrect footprint will likely require re-placing entire sections of the board.

If you are working from established libraries you won’t have to worry much about incorrect footprints. The problem is that many new designs are based on new components which will mean new footprints in your PCB libraries. Often these new footprints will be in a preliminary state until they are finalized and their shapes may end up changing. An important item to include in your checklist is if the new footprints have been updated in the library and if the schematic has been correctly updated to point to these new footprints. You will also want to check to make sure that any footprint modifications for specific design technologies (like RF), or specialized parts (like dual footprints) have been incorporated.

Make sure that your library footprints are correct and up-to-date

Is the Schematic Far Enough Along for Placement?

The schematic will probably not be finalized until right before the board design is completed, but you should know how far along it is before you begin placement. Having to make design changes to accommodate new circuitry is part of the game, but having to completely redesign the board is a different story. All too often I have had to throw away significant portions of my placement and start over because the schematic completely changed, and no one thought that it was important enough for me to know that.

To avoid these kinds of frustrations, include some of the following in your pre-placement checklist:

Are you anticipating adding or removing components to this design?

Are there any components in this design that might be changed to a different footprint?

Are you expecting any circuitry changes in this design that might force placement changes, such as changes to signal paths?

With questions like these, you can get a much better idea of how far along the schematic is before you start placement. Remember that the key isn’t having the schematic totally done, it is in understanding how far along the schematic is and what may or may not change as you go. This way you can be prepared for the changes when they do come.

Once your pre-placement checklist is complete, you will be ready to place those components

Do You Have the Placement Information to Close the Pre-Placement Checklist?

Once you have confirmed that the library and the schematic are ready, or mostly ready for placement, you need to finalize all of the component placement information. Use the following list for your pre-placement checklist:

Has the board outline been finalized?

Has the position of fixed components such as connectors, switches, or shields been finalized?

Are there any thermal concerns that will impact placement?

Are there any placement requirements for power distribution?

Are there any placement requirements for critical circuitry?

Some of these items may be automatically taken care of by your design process if you receive your board outline information from a mechanical CAD system. In that case, the board outline and the positions of your fixed components will probably be specified to match up with any features of what the board will be enclosed in. It is still best though to include these kinds of questions in your pre-placement checklist to make sure that you have all of your bases covered, and to remind yourself of what exactly needs to be ensured.

By creating and following a pre-placement checklist, you will give yourself the best chance to place your board without having to go back and re-do the placement later to correct errors. There are many other areas where a checklist can be useful, such asbefore starting your layout and when youcreate the final deliverable output files. The old adage in construction is to measure twice and cut once; for those of us designing PCBs, it might be better stated as check twice, and place once.